1. Field of the Invention
This invention relates to a quantizing error reducer for an audio signal, which is constructed to reduce a quantizing error generated in quantization of an audio signal.
2. Description of the Prior Art
At present, as digital audio equipments for handling audio signals in a digital form, there exist, e.g., players for so called compact discs (CD) and/or so called digital audio tape recorders (DATs), etc. In these digital audio equipments, various unified standard requirements are prescribed. For example, in the case of the bit length of digital audio signals handled in these equipments, it is prescribed from the above unified standard requirements that such audio signals have a bit length of 16 bits. Further, as the digital audio signals in these digital audio equipments, there are used digital audio signals obtained by encoding an analog audio signals (sound waveform signals) by using a simple linear quantization like, e.g., so called PCM (linear pulse coding).
Meanwhile, in recent years, in digital audio equipments as described above, it is required from the above-mentioned unified standard requirements that there is provided a reproduced sound having a quantity higher, in view of sense of hearing, than that of a reproduced sound actually obtained. In order that such a reproduced sound more excellent in view of hearing sense is provided, a scheme is conceivable to allow digital audio signals handled in these digital audio equipments to be signals in which noise components included in the digital audio signals themselves are reduced. Reproduced sounds obtained from digital audio signals in which their noise components are reduced in this way have less noise.
As the processing for reducing the noise component of the digital audio signal, there is known, e.g., a processing system to reduce a quantizing error by so called an error feedback to feed a quantizing error (quantizing noise or quantizing distortion) generated by a quantizer in quantizing an audio signal back to the input side of the quantizer through a noise filter.
Here, the quantizing distortion in the linear quantization like the above-described PCM coding has a frequency characteristic flat over the entire frequency band of an audio signal. However, since the ear of the human being has a difference in the hearing sensitivity depending upon a frequency of sound, it cannot be said that the quantizing error reducing processing by the error feedback is necessarily effective in view of hearing sense.
This invention has been proposed with the actual circumstances as described in view, and its object is to provide a quantizing error reducer for an audio signal, which is capable of reducing a quantizing error (quantizing noise) effectively from a viewpoint of hearing sense.
A quantizing error reducer according to this invention has been proposed in order to attain the above-described object. In this quantizing error reducer, a quantizing error generated in a quantizer is fed back to the input side of the quantizer through a noise filter. The filter coefficient of the noise filter is set on the basis of information relating to an equi-loudness curve corresponding to the hearing sense characteristic of the human being.
In accordance with this invention, by drawing attention to the fact that the equi-loudness curve is in correspondence with the hearing sense characteristic of the human being, an approach is employed to feed a quantizing error back to the input side of the quantizer through the noise filter of which coefficient is set on the basis of information relating to the equi-loudness curve, thereby making it possible to reduce a quantizing error in a frequency band of a sound easy to be heard. Accordingly, it is possible to reduce noises in the hearing sense thus to improve the dynamic range in the hearing sense.